In the art of web conveyance, venting of air entrained between a web and a conveyance roller is needed to maintain contact between the web and the roller. Referring to FIG. 1A, a typical conveyance roller 1 with a web 2 trained around a portion of the roller surface 4 is illustrated. As the speed of the web 2 increases, the entrained air partially or wholly lifts the web 2 off the roller surface 4 which allows relative motion between the web 2 and the roller surface 4 to develop. This relative motion often results in damage (such as scratching) to the web 2 and associated machine conveyance problems (such as poor tracking).
Referring again to FIG. 1A, it is well known that air entrainment occurs when the boundary layer of air that travels with both the web 2 and roller 1 is drawn into the web-to-roller interface 5. There are several existing roller surface technologies that have addressed, to some extent, venting of entrained air and the associated surface defects that arise if the air is not sufficiently vented. These roller surface technologies include: (1) grooved rollers described, for instance, in U.S. Pat. No. 3,405,855 by Daley et al., Oct. 15, 1968, titled “Paper Guide and Drive Roll Assemblies”; (2) grit-blasted rollers, disclosed, for instance, in U.S. Pat. No. 4,426,757 by Hourticolon et al., Jan. 24, 1984, titled “Web Guide Roller For Use At High Speeds And Process For Producing The Same;” and, (3) shot blasted rollers described, for instance, in U.S. Pat. No. 4,910,843 by Lioy et al, Mar. 27, 1990, titled “A Process For Finishing The Surface Of A Roller.” Experience has shown that each of these technologies can and do cause surface defects such as surface impressions in the web that mirror the roller surface venting pattern and/or roller surface roughness. These problems are particularly worrisome on highly sensitive webs, such as photosensitive film and paper, in areas of the web conveyance machine where the web or coatings are sensitive to marking. Particular examples include surface impression defects that can occur in the manufacture of soft webs like acetate, having a high solvent content or Poly(ethylene terephthalate) (PET), near or above the glass transition temperature (Tg).
Referring to FIGS. 1B and 2B, a highly polished roller surface 4 used in most web conveyance devices (not shown) has a typical surface roughness characterized by an Ra value of less than 7 micro-inches (FIGS. 3A and 3B). This level of surface roughness minimizes the sharp up features that can damage the web 2 or coatings due to impressions or minute scratches. Commonly referred to as honing, this defect is caused by web dimensional changes during contact with a roller surface 4. FIG. 9 illustrates an image of a web conveyed on a typical prior art roller which produced a honing defect 6 typically caused by a smooth, grit-blasted or shot-blasted roller.
Referring again to FIGS. 1B and 2B, experience has shown that the polished roller 4 (described above) can cause a larger scratch defect in the conveyed web at tensions less than 175 N/m and at web speeds above 15 m/min. These larger scratches are caused by large relative motion between the web 2 and roller surface 4 due to loss of traction between the web 2 and roller surface 4 caused by air entrainment. Experience has also shown that using conventional roller surface technology (e.g. grooved rollers, grit-blasted rollers, and plateau honed shot-blasted rollers) to vent the entrained air prevents the larger scratches caused by loss of traction. However, the minute defects (e.g. impressions and honing) are worse than they were on the highly polished rollers.
With reference to FIG. 1A, grit blasted rollers provide air venting because the rough surface provides areas for the entrained air to go without lifting the web 2 away from the roller surface 4. However, the grit blasting process produces sharp up features that cause impressions and honing. Polishing the surface 4 still leaves sharp up features and impressions, and honing problems remain. Shot-blasted rollers also have sharp up features due to the nature of the manufacturing process. Again, it is our experience that grinding the roller surface 4 still leaves sharp up features and impressions, and honing problems remain. Moreover, it is our experience that grooved rollers, even ones with well-rounded groove comers and lands with surface roughness characterized by an Ra less than 5 micro-inches, cause impressions and honing.
Referring again to FIG. 1A, another problem common to typical high-traction conveyance rollers is the collection of debris that adheres tenaciously to the roller surface 4. This debris may come from the web 2 or from the environment and cause the same defects as sharp up features on the roller surface 4 as described previously.
A commonly accepted solution to the aforementioned problem is to run higher tensions on smooth rollers (tensions up to 500 N/m) to prevent loss of traction due to air entrainment at speeds up to 150 m/min. However, higher tensions are not generally desirable because they can make impression defects worse. Moreover, high tension can also cause orientations of the polymer chains that are undesirable (e.g. triacetate retardation, polyester heat relax). Furthermore, the aforementioned solution cannot be used everywhere in web conveyance devices because rollers with different surface finishes are used depending on the process conditions in a particular machine section.
Therefore, a need persists in the art for a web conveyance roller that maximizes venting of entrained air and that has a surface finish that resists the formation of markings on the conveyed web, such as impression and honing defects.